1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, or facsimile of, for example, an electrophotography system or electrostatic recording system.
2. Description of the Related Art
Conventionally, a color image forming apparatus, which forms an image for one color by an image forming unit including a pair of laser scanners and an image bearing member, and includes image forming units as many as the number of colors, is known. In this arrangement, laser light beams, which are deflected and scanned by rotary polygon mirrors arranged for respective colors, are detected by laser detectors (to be referred to as Beam Detection (BD) sensors hereinafter) for respective colors, so as to generate horizontal sync signals (to be referred to as BD signals hereinafter) required to detect image write start timings of respective colors. Then, rotation cycles (to be referred to as BD cycles hereinafter) of scanner motors of respective colors are detected based on the BD signals. Also, emission (to be referred to as force emission hereinafter) of a semiconductor laser is performed until the BD cycles reach a predetermined value, and respective image bearing members are irradiated with laser light beams of respective colors reflected by the rotary polygon mirrors using a plurality of reflection surfaces.
Also, as a more inexpensive arrangement, a color image forming apparatus including one laser scanner and a plurality of image bearing members is known. In this arrangement, a single rotary polygon mirror, which is driven by a single scanner motor, is irradiated with laser light beams of respective colors, and the respective image bearing members are irradiated with the laser light beams using a plurality of reflection surfaces.
For example, Japanese Patent Application Laid-Open No. H04-313776 has proposed an arrangement in which a plurality of light sources is simultaneously scanned on a photosensitive member by different surfaces of a polygon mirror. Then, it proposes a unit which determines BD signals of respective colors of remaining light sources besides a light source provided with a BD sensor from a BD signal of the light source provided with the BD sensor and the rotation phase differences of the surfaces of the polygon mirror. For example, Japanese Patent Application Laid-Open No. 2004-345171 has proposed the following arrangement so as to solve color misregistration at transfer timings of respective colors caused by surface-division precisions of respective mirror surfaces of a rotary polygon mirror, which do not completely match. That is, this arrangement includes one BD sensor, which receives reflected laser light from the rotary polygon mirror, a BD signal output unit, which outputs a BD signal at a reception timing of the reflected laser light, and a pseudo BD sensor signal generation unit, which generates a pseudo BD sensor signal as a pseudo signal after an elapse of a predetermined period of time from the BD sensor. Then, a reference signal of a write start position in a main scanning direction of laser light with which a surface different from that irradiated with laser light, with which the BD sensor is irradiated, is irradiated with and reflected is defined by the pseudo BD sensor signal generation unit. Since Japanese Patent Application Laid-Open No. 2004-345171 includes such arrangement, misregistration at transfer timings of respective colors can be reduced even by one BD sensor.
In Japanese Patent Application Laid-Open No. H04-313776, the following control is executed. That is, during a period until a scanner motor is rotated at a stable speed (to be referred to as a scanner start-up period hereinafter), semiconductor lasers of respective colors emit laser light beams so as to detect a BD cycle and to calculate a rotation number of the scanner motor. Then, the semiconductor lasers of respective colors emit laser light beams, and light amount adjustment of the semiconductor lasers of respective colors is also executed at that timing. At this time, in a station in which force emission is performed, a photosensitive member surface is irradiated with laser light to change a potential of the photosensitive member surface. For this reason, control for determining a voltage required to supply a predetermined current to a transfer unit at an image formation timing (to be referred to as Active Transfer Voltage Control (ATVC) control hereinafter) cannot be executed during the scanner start-up period. As a result, a first print-out time (FPOT) from a print instruction until printing of the first page is completed cannot be shortened.